The technique of in-mould labelling (IML) has been known for many years. It involves the use of paper or plastic labels which ultimately form an integral part of the moulded product. The in-mould labels must, therefore, be able to tolerate the heat applied during the moulding process. The resultant product is a pre-decorated item, such as a container or the like, which may be filled thereafter. In contrast to glue applied or pressure-sensitive labels which appear above the surface of the container, in-mould labels appear as part of the container. Effectively, in-mould labelling eliminates the need for a separate labelling process following the manufacture of the container, which reduces labour and equipment costs.
In-mould labels generally comprise a carrier base, consisting of a polymeric or cellulosic carrier film, on which a decorative pattern or a written message is printed. The thus obtained label is subsequently positioned against a wall of a mould for injection moulding or for blow moulding or the like, held in place by various means, such as electrostatic forces or vacuum suction, and a polymeric article is moulded by injecting a mass of polymeric melt or by blowing a polymeric parison against the mould walls on which the in-mould label is applied. This causes the label to join the moulded article and can be regarded as an integral part of it. The adhesion of such labels to the polymeric article can be enhanced by applying a heat sealable layer (a film or a coating) onto the backing side (i.e., not printed surface) of the in-mould label which is to be in contact with the polymeric article.
In-mould labels can be used to cover a portion of a container or to cover the entire outer surface of a container. In the latter case, the in-mould label serves as an additional layer and may, therefore, enhance the structural integrity of the container.
During the moulding of certain articles, for example container lids or parallel sided containers, film shrinkage can cause distortion of the label and/or the moulded article. For example, this distortion may result in a warping or bending effect of the article and is highly undesirable. In extreme cases, distortion of this type can result in poorly fitting components, e.g. lids on containers, or poor ‘nesting’ of multiple containers.
Conventionally, the problem of distortion is particularly acute when labels formed of solid, biaxially oriented polypropylene films are subjected to in-mould labelling techniques. As a result, cast polypropylene or cavitated biaxially oriented polypropylene films are used as in-mould label substrates.
However, the use of these materials results in further disadvantages. For example, cast polypropylene is a low stiffness material and therefore labels comprising this material as a substrate must have increased thickness to provide acceptable structural properties as compared to biaxially oriented polypropylene in-mould labels. Conversion and moulding with cast polypropylene is also considered to be inferior to biaxially oriented polypropylene in terms of consistency, resulting in reduced output and production efficiency.
Additionally, cavitated biaxially oriented polypropylene converts well but results in a matt effect label due to collapse of the cavitated structure.
Producing gloss effect lids and parallel sided containers at high efficiencies and low cost from conventional materials is therefore challenging. The provision of an in-mould label which simultaneously exhibits stiffness comparable or greater than that of biaxially oriented polypropylene, distortion which is comparable to or lower than that exhibited by cast polypropylene or cavitated oriented polypropylene, and also high clarity would be of great value.